/**
 * Copyright (C) 2002-2015   The FreeCol Team
 * <p>
 * This file is part of FreeCol.
 * <p>
 * FreeCol is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 2 of the License, or
 * (at your option) any later version.
 * <p>
 * FreeCol is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * <p>
 * You should have received a copy of the GNU General Public License
 * along with FreeCol.  If not, see <http://www.gnu.org/licenses/>.
 */

package net.sf.freecol.server.ai.mission;

import java.util.List;
import java.util.logging.Logger;

import javax.xml.stream.XMLStreamException;

import net.sf.freecol.common.io.FreeColXMLReader;
import net.sf.freecol.common.io.FreeColXMLWriter;
import net.sf.freecol.common.model.Colony;
import net.sf.freecol.common.model.CombatModel;
import net.sf.freecol.common.model.Location;
import net.sf.freecol.common.model.Direction;
import net.sf.freecol.common.model.PathNode;
import net.sf.freecol.common.model.Settlement;
import net.sf.freecol.common.model.Tile;
import net.sf.freecol.common.model.Unit;
import net.sf.freecol.common.model.Unit.UnitState;
import net.sf.freecol.common.model.pathfinding.CostDeciders;
import net.sf.freecol.common.model.pathfinding.GoalDecider;
import net.sf.freecol.common.util.LogBuilder;
import net.sf.freecol.server.ai.AIMain;
import net.sf.freecol.server.ai.AIMessage;
import net.sf.freecol.server.ai.AIUnit;


/**
 * Mission for defending a <code>Settlement</code>.
 */
public class DefendSettlementMission extends Mission
{

    private static final Logger logger = Logger.getLogger( DefendSettlementMission.class.getName( ) );

    /** The tag for this mission. */
    private final String tag = "AI defender";

    /** Maximum number of turns to travel to the settlement. */
    private static final int MAX_TURNS = 20;

    /** The settlement to be protected. */
    private Location target;


    /**
     * Creates a mission for the given <code>AIUnit</code>.
     *
     * @param aiMain The main AI-object.
     * @param aiUnit The <code>AIUnit</code> this mission
     *        is created for.
     * @param settlement The <code>Settlement</code> to defend.
     */
    public DefendSettlementMission( AIMain aiMain, AIUnit aiUnit,
                                    Settlement settlement )
    {
        super( aiMain, aiUnit, settlement );
    }

    /**
     * Creates a new <code>DefendSettlementMission</code> and reads
     * the given element.
     *
     * @param aiMain The main AI-object.
     * @param aiUnit The <code>AIUnit</code> this mission is created for.
     * @param xr The input stream containing the XML.
     * @throws XMLStreamException if a problem was encountered
     *      during parsing.
     * @see net.sf.freecol.server.ai.AIObject#readFromXML
     */
    public DefendSettlementMission( AIMain aiMain, AIUnit aiUnit,
                                    FreeColXMLReader xr ) throws XMLStreamException
    {
        super( aiMain, aiUnit );

        readFromXML( xr );
    }


    /**
     * Extract a valid target for this mission from a path.
     *
     * @param aiUnit The <code>AIUnit</code> to perform the mission.
     * @param path A <code>PathNode</code> to extract a target from.
     * @return A target for a <code>DefendSettlementMission</code>, or null
     *     if none found.
     */
    public static Location extractTarget( AIUnit aiUnit, PathNode path )
    {
        if ( path == null ) return null;
        final Location loc = path.getLastNode( ).getLocation( );
        Settlement settlement = loc.getSettlement( );
        return ( invalidReason( aiUnit, settlement ) == null ) ? settlement
                : null;
    }

    /**
     * Evaluate allocating a unit to the defence of a settlement.
     *
     * @param aiUnit The <code>AIUnit</code> that is to defend.
     * @param path A <code>PathNode</code> to take to the settlement.
     * @return A value for such a mission.
     */
    public static int scorePath( AIUnit aiUnit, PathNode path )
    {
        final Location loc = extractTarget( aiUnit, path );
        return ( loc instanceof Settlement )
                ? aiUnit.getAIOwner( ).adjustMission( aiUnit, path,
                                                      DefendSettlementMission.class,
                                                      ( int ) ( 1000 * ( ( Settlement ) loc ).getDefenceRatio( )
                                                              / ( path.getTotalTurns( ) + 1 ) ) )
                : Integer.MIN_VALUE;
    }

    /**
     * Gets a <code>GoalDecider</code> for finding the best colony
     * <code>Tile</code>, optionally falling back to the nearest colony.
     *
     * @param aiUnit The <code>AIUnit</code> that is searching.
     * @return A suitable <code>GoalDecider</code>.
     */
    private static GoalDecider getGoalDecider( final AIUnit aiUnit )
    {
        return new GoalDecider( )
        {
            private PathNode bestPath = null;
            private int bestValue = Integer.MIN_VALUE;

            @Override
            public PathNode getGoal( ) { return bestPath; }

            @Override
            public boolean hasSubGoals( ) { return true; }

            @Override
            public boolean check( Unit u, PathNode path )
            {
                int value = scorePath( aiUnit, path );
                if ( bestValue < value )
                {
                    bestValue = value;
                    bestPath = path;
                    return true;
                }
                return false;
            }
        };
    }

    /**
     * Finds a path to the best nearby settlement to defend.
     *
     * @param aiUnit The <code>AIUnit</code> to execute this mission.
     * @param range An upper bound on the number of moves.
     * @param deferOK Enables deferring to a fallback colony.
     * @return A path to the new target, or null if none found.
     */
    public static PathNode findTargetPath( AIUnit aiUnit, int range,
                                           boolean deferOK )
    {
        if ( invalidAIUnitReason( aiUnit ) != null ) return null;
        final Unit unit = aiUnit.getUnit( );
        final Location start = unit.getPathStartLocation( );

        return unit.search( start, getGoalDecider( aiUnit ),
                            CostDeciders.avoidSettlementsAndBlockingUnits( ),
                            range, unit.getCarrier( ) );
    }

    /**
     * Finds a path to the best nearby settlement to defend.
     *
     * @param aiUnit The <code>AIUnit</code> that is searching.
     * @param range An upper bound on the number of moves.
     * @param deferOK Enables deferral (not implemented in this mission).
     * @return A suitable target, or null if none found.
     */
    public static Location findTarget( AIUnit aiUnit, int range,
                                       boolean deferOK )
    {
        PathNode path = findTargetPath( aiUnit, range, deferOK );
        return ( path != null ) ? extractTarget( aiUnit, path ) : null;
    }

    /**
     * Why would a DefendSettlementMission be invalid with the given unit?
     *
     * @param aiUnit The <code>AIUnit</code> to test.
     * @return A reason why the mission would be invalid with the unit,
     *     or null if none found.
     */
    private static String invalidMissionReason( AIUnit aiUnit )
    {
        String reason = invalidAIUnitReason( aiUnit );
        if ( reason != null ) return reason;
        final Unit unit = aiUnit.getUnit( );
        final CombatModel cm = unit.getGame( ).getCombatModel( );
        return ( cm.getDefencePower( null, unit ) <= 0.0f ) ? "unit-not-defender"
                : null;
    }

    /**
     * Why is this mission invalid with a given settlement target?
     *
     * @param aiUnit The <code>AIUnit</code> to check.
     * @param settlement The potential target <code>Settlement</code>.
     * @return A reason for mission invalidity, or null if none found.
     */
    private static String invalidSettlementReason( AIUnit aiUnit,
                                                   Settlement settlement )
    {
        return invalidTargetReason( settlement, aiUnit.getUnit( ).getOwner( ) );
    }

    /**
     * Why would this mission be invalid with the given AI unit?
     *
     * @param aiUnit The <code>AIUnit</code> to check.
     * @return A reason for mission invalidity, or null if none found.
     */
    public static String invalidReason( AIUnit aiUnit )
    {
        String reason = invalidMissionReason( aiUnit );
        return ( reason != null )
                ? reason
                : ( aiUnit.getUnit( ).getOwner( ).getNumberOfSettlements( ) <= 0 )
                ? Mission.TARGETNOTFOUND
                : null;
    }

    /**
     * Why would this mission be invalid with the given AI unit and location?
     *
     * @param aiUnit The <code>AIUnit</code> to check.
     * @param loc The <code>Location</code> to check.
     * @return A reason for invalidity, or null if none found.
     */
    public static String invalidReason( AIUnit aiUnit, Location loc )
    {
        String reason = invalidMissionReason( aiUnit );
        return ( reason != null )
                ? reason
                : ( loc instanceof Settlement )
                ? invalidSettlementReason( aiUnit, ( Settlement ) loc )
                : Mission.TARGETINVALID;
    }


    // Implement Mission
    //   Inherit dispose, getTransportDestination, isOneTime

    /**
     * {@inheritDoc}
     */
    @Override
    public int getBaseTransportPriority( )
    {
        return ( getTransportDestination( ) == null ) ? 0
                : NORMAL_TRANSPORT_PRIORITY + 5;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public Location getTarget( )
    {
        return target;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void setTarget( Location target )
    {
        if ( target == null || target instanceof Settlement )
        {
            this.target = target;
        }
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public Location findTarget( )
    {
        return findTarget( getAIUnit( ), 4, true );
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public String invalidReason( )
    {
        return invalidReason( getAIUnit( ), getTarget( ) );
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public Mission doMission( LogBuilder lb )
    {
        lb.add( tag );
        String reason = invalidReason( );
        if ( isTargetReason( reason ) )
        {
            return retargetMission( reason, lb );
        }
        else if ( reason != null )
        {
            return lbFail( lb, false, reason );
        }

        // Go to the target!
        final Unit unit = getUnit( );
        Unit.MoveType mt = travelToTarget( getTarget( ),
                                           CostDeciders.avoidSettlementsAndBlockingUnits( ), lb );
        switch ( mt )
        {
            case MOVE: // Arrived
                break;

            case MOVE_HIGH_SEAS:
            case MOVE_NO_MOVES:
            case MOVE_NO_REPAIR:
            case MOVE_ILLEGAL:
                return lbWait( lb );

            case MOVE_NO_ACCESS_EMBARK:
            case MOVE_NO_TILE:
                return this;

            default:
                return lbMove( lb, mt );
        }

        // Check if the mission should change?
        // Change to supporting the settlement if the size is marginal.
        final AIMain aiMain = getAIMain( );
        final AIUnit aiUnit = getAIUnit( );
        Mission m = null;
        if ( getTarget( ) instanceof Colony )
        {
            Colony colony = ( Colony ) getTarget( );
            if ( unit.isInColony( )
                    || ( unit.isPerson( ) && colony.getUnitCount( ) < 1 ) )
            {
                m = getEuropeanAIPlayer( ).getWorkInsideColonyMission( aiUnit,
                                                                       aiMain.getAIColony( colony ) );
                return lbDone( lb, m != null, "bolster ", colony );
            }
        }

        // Anything more to do?
        switch ( unit.getState( ) )
        {
            case FORTIFIED:
                return lbWait( lb, ", fortified" );
            case FORTIFYING:
                return lbWait( lb, ", fortifying" );
            default:
                break;
        }

        // Check if the settlement is badly defended.  If so, try to fortify.
        Settlement settlement = ( Settlement ) getTarget( );
        int defenderCount = 0, fortifyCount = 0;
        List< Unit > units = settlement.getUnitList( );
        units.addAll( settlement.getTile( ).getUnitList( ) );
        for ( Unit u : units )
        {
            AIUnit aiu = getAIMain( ).getAIUnit( u );
            if ( invalidMissionReason( aiu ) == null )
            {
                defenderCount++;
                switch ( u.getState( ) )
                {
                    case FORTIFIED:
                    case FORTIFYING:
                        fortifyCount++; break;
                    default:
                        break;
                }
            }
        }
        if ( defenderCount <= 2 || fortifyCount <= 1 )
        {
            return ( ! unit.checkSetState( UnitState.FORTIFYING ) )
                    ? lbWait( lb, ", waiting to fortify at ", settlement )
                    : ( AIMessage.askChangeState( aiUnit, UnitState.FORTIFYING )
                    && unit.getState( ) == UnitState.FORTIFYING )
                    ? lbWait( lb, ", now fortifying at ", settlement )
                    : lbFail( lb, false, ", fortify failed at ", settlement );
        }

        // The target is well enough defended.  See if the unit
        // should attack a nearby hostile unit.  Remember to prevent a
        // sole unit attacking because if it loses, the settlement
        // will collapse (and the combat model does not handle that).
        if ( ! unit.isOffensiveUnit( ) )
        {
            return lbFail( lb, false, "not-offensive-unit" );
        }

        final CombatModel cm = unit.getGame( ).getCombatModel( );
        Unit bestTarget = null;
        double bestDifference = Double.MIN_VALUE;
        Direction bestDirection = null;
        for ( Direction d : Direction.getRandomDirections( "defendSettlements",
                                                           logger, getAIRandom( ) ) )
        {
            Tile t = unit.getTile( ).getNeighbourOrNull( d );
            if ( t == null ) continue;
            Unit defender = t.getFirstUnit( );
            if ( defender != null
                    && defender.getOwner( ).atWarWith( unit.getOwner( ) )
                    && unit.getMoveType( d ).isAttack( ) )
            {
                Unit enemyUnit = t.getDefendingUnit( unit );
                double enemyAttack = cm.getOffencePower( enemyUnit, unit );
                double weAttack = cm.getOffencePower( unit, enemyUnit );
                double enemyDefend = cm.getDefencePower( unit, enemyUnit );
                double weDefend = cm.getDefencePower( enemyUnit, unit );
                double difference = weAttack / ( weAttack + enemyDefend )
                        - enemyAttack / ( enemyAttack + weDefend );
                if ( difference > bestDifference )
                {
                    if ( difference > 0 || weAttack > enemyDefend )
                    {
                        bestDifference = difference;
                        bestTarget = enemyUnit;
                        bestDirection = d;
                    }
                }
            }
        }

        // Attack if a target is available.
        if ( bestTarget != null )
        {
            AIMessage.askAttack( getAIUnit( ), bestDirection );
            return lbAttack( lb, bestTarget );
        }

        return lbWait( lb, ", alert at ", getTarget( ) );
    }


    // Serialization

    private static final String SETTLEMENT_TAG = "settlement";


    /**
     * {@inheritDoc}
     */
    @Override
    protected void writeAttributes( FreeColXMLWriter xw ) throws XMLStreamException
    {
        super.writeAttributes( xw );

        if ( target != null )
        {
            xw.writeAttribute( SETTLEMENT_TAG, target.getId( ) );
        }
    }

    /**
     * {@inheritDoc}
     */
    @Override
    protected void readAttributes( FreeColXMLReader xr ) throws XMLStreamException
    {
        super.readAttributes( xr );

        target = xr.getAttribute( getGame( ), SETTLEMENT_TAG,
                                  Settlement.class, ( Settlement ) null );
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public String getXMLTagName( ) { return getXMLElementTagName( ); }

    /**
     * Gets the tag name of the root element representing this object.
     *
     * @return "defendSettlementMission".
     */
    public static String getXMLElementTagName( )
    {
        return "defendSettlementMission";
    }
}
